Abstract: Ultra-high Performance Seawater and Sea-sand concrete represents a highly promising material for marine engineering construction. By directly utilizing abundant seawater and seasand resources, it not only alleviates the scarcity of freshwater and river sand in marine projects but also effectively mitigates chloride ion corrosion through its inherently dense microstructure, thereby enhancing structural durability. However, its inherent high brittleness has somewhat limited its application. To overcome this limitation, this study aims to enhance its toughness by introducing stainless steel fibers, systematically investigating the synergistic effects of water-to-binder ratio and fiber content on the mechanical properties of UHPSSC. Results indicate that a water-to-binder ratio of 0.20 and a stainless steel fiber volume content of 2% represent the optimal mix design among the nine tested formulations. This optimal mix design increased the axial compressive strength of UHPSSC by 22.57% and flexural strength by 46.32%, with a flow value of 260 mm, elastic modulus of 41.17 GPa, and brittleness coefficient of 6.12. Therefore, the preparation of stainless steel fiber-reinforced ultra-high performance seawater sea sand concrete with excellent mechanical properties in this study not only provides an effective solution to the challenge of chloride ion corrosion in seawater sea sand but also lays a crucial theoretical and experimental foundation for the direct and high-value utilization of marine resources.